One of the main applications of Wire Electric Discharge Machines (WEDM), representing a major part of the entire WEDM application range is the production of punches and dies for stamping tools, whereas punch and die have essentially same geometry, but slightly different size. By cutting a closed contour in a work piece, for instance a die, a core part remains. When a die is cut, the produced core piece (so called dropout part, slug, cutout) is not used and has to be removed because it is an obstacle for the further processing of the die. Similarly, if the core piece is the desired part to be manufactured, it has to be extracted carefully and safely stored for further processing.
Traditionally the main cut of a contour is cut leaving a small connection joint, so that the core part remains suspended in the work piece. The machine stops, and the operators fixes the core part by means of magnets or by gluing or by otherwise holding in place the core part. Then the operator restarts the control program and the machine cuts the small connection joint. The machine stops again, and the operators removes the one or more core parts. Finally he restarts the control program and the machine completes the dies by executing one or more trim cuts.
One may decide to simply let the dropout part fall into the work tank or into a collection basket by gravity, which however is risky. In fact a small core part may slide down into the nozzle or adhere to the work piece and thus cause an uncertain condition, with an increased risk of collision and possible damage of the work piece and the lower nozzle or other machine parts. On the other hand, a large dropout part or a plurality of slugs at the bottom of the work tank may be of hindrance, and there is an increased risk of collision with the lower arm. To reduce the risk a larger dropout part may be cut in smaller sections.
If the size of the core part is small, a dropout part can be avoided by a so-called no-core cut (also full-removal cut or pocketing), by which the entire core part is completely eroded. This technique can also be used to eliminate protruding sections of a dropout part, which may be of hindrance to extract the dropout part. The pocketing technique has the advantage that no additional handling devices are required, and there remains no waste piece to be handled. However it is time consuming, and generally it is only reasonable with comparably small slug volumes. The control may thus be instructed to adopt the full-removal cut strategy based on the volume of the dropout part and the expected machining time for the full-removal cut.
Among the WEDM in the field many are integrated in a communication network by which they can be monitored and controlled remotely, and some comprise pallet changers with identification chips, extra-large wire spool sizes or double wire circuit, central filter station for the conditioning of the dielectric etc., by which the autonomous, unattended operation can be substantially extended. However, still today most of the machines have no automatic means for the removal of the dropout part in WEDM, which are thus mainly removed manually after the main cut or are simply made to drop in the work tank. To slightly relieve this situation, some control devices comprise so called “early-late” strategies (EP 512 314), by which the operator can organize the machining sequence of one or more pieces to be cut in function of his presence at the machine. The operator can select the time for actual intervention in the generally lengthy operating process by organizing the sequence of the main cut (full cut), the separation cut and the trim cuts. He can adapt the time for this intervention according to his labor time or more generally, according his availability. These strategies are advantageous, since the entire processing time in WEDM is very long compared with the time needed for the separation cut and for the manual intervention, and because essentially no additional hardware is required.
Beside the above mentioned measures which increase the availability of the machine and user-friendliness by simple improvement of the control, a number of devices for the automatic removal of the dropout part have been proposed in patent literature, but only few have been implemented in practice.
As an example, in the late eighties an attempt has been made with the mechanical dropout part remover Agiepick. This device was able to remove a dropout piece having a size of 10 mm up to about 50 mm square. The core part to be removed was grasped by means of a moveable arm having an expandable pin. The core part comprises an appropriate hole of 3 mm diameter in which the expandable pin was inserted after the completion of the separation cut. Then the expandable pin was expanded and the core part was lifted by means of a vertical lifting movement and then moved laterally by a rotational movement. Here the expandable pin was loosened so that the core part dropped in a receptacle. A detection plate was incorporated to provide a control signal. The hole was either the start hole or an additional hole made for the particular purpose of core part removal. The mechanical picking device was coupled with the hole of the dropout piece by force closure. The expandable pin was inserted in the comparably small hole, and needed to be centered accurately. Thus the expandable pin was subject to wear and deterioration; erroneous positioning and manipulation errors lead inevitably to malfunctioning and failure of the dropout part remover, so that the primary scope was not achieved reliably.
Another solution has been proposed with the Eject 1000, in which the control program generates a connection joint formed as a triangular nose. After the main cut the slug is separated and ejected by a hammer-like mechanism.
As a further example, a collecting basket mounted with the lower wire guiding arm has been realized. This solution is comparably simple but fits only for certain applications, with comparably small parts, and limits the axis stroke due to the encumbrance of the collecting basket.
Other mechanical solutions have been described in the patent literature, f.i. in JP2002-001618A, in which a pin array actuated from the lower side of the work piece presses against the lower surface to lift the core, which is then grasped by a gripper. In general mechanical dropout part removers are all somehow critical from the reliability point of view. Thus other principles have been considered to hold the slug:                Electromagnetic devices such as the ones described f.i. in JP05-269625A, JP04-310316A, JP04-063629A, JP03-294118A, JP02-131816A, JP63-185531A, EP194353A1 are easily controllable, but have the disadvantage that they can be used only with ferromagnetic work pieces. JP03-256617A is a combination of an electromagnetic device and a suction pad.        Suction devices such as the ones described f.i. in JP03-256616A, JP03-213214A, JP03-079225A, EP194353A1, JP60-180726A are of comparably simple constitution, and negative pressure is generally available on a WEDM, and works with all materials and most surfaces, but have some disadvantages which will be discussed later on.        Sometimes the slug removing devices are supported by fluid pressure jet acting on the lower surface of the slug, as shown in JP08-001439A, JP03-294118A, JP03-270822A.        
As known the size of slugs and their geometries, as well as the range of materials to be processed by WEDM is wide. The known devices are not satisfying. Each has one or more drawbacks, but the most important aspect is their unreliability and low applicability to machine environment.
The scope of the present invention is thus to provide a simple device for the reliable automatic removal of core pieces. The foregoing and other objects and advantages will become apparent from the detailed description of the invention and the accompanying drawings.